1. Field of the Invention
The present invention relates to antenna systems and, in particular, to an antenna system that uses a multi-element antenna array to receive an electromagnetic signal and generate a plurality of electrical signals that are representative of the received electromagnetic signal and a multi-beam beamformer to produce at least two spatially independent, overlapping beams from the electrical signals provided by the array.
2. Description of the Related Art
Presently, antenna systems are used in many aeronautical applications. One application for an antenna system is as part of a radar on an aircraft where the antenna system is used to obtain positional information on the objects in the space surrounding the aircraft. This positional information is then typically used to either avoid an object, such as another aircraft, or to home in on an object, such as a navigational beacon.
Present radar antenna system technology is becoming increasingly hard-pressed to provide sufficient information on the space around an aircraft as the speed of the aircraft on which the radar antenna system is mounted and of the other aircraft in the surrounding airspace increases. Specifically, present radar antenna systems are generally proving inadequate with respect to their detection range, field of view, resolution, or combinations thereof in situations that involve high-speed aircraft. For example, if the radar antenna system in one of two high-speed aircraft that are approaching one another has a short range, the radar antenna system may not be able to provide information on the other aircraft in time to avoid a collision. If, on the other hand, the radar antenna system in an aircraft has a limited area of coverage, and the aircraft is attempting to join up with a high-speed aircraft, the high-speed aircraft may not be detected with such a radar antenna system. Further, if the antenna system in an aircraft has a limited resolution, the ability of the aircraft to make the appropriate course corrections may be compromised.
Further, high-speed aircraft also exhibit high skin temperatures that are on the order of 3000xc2x0 F. and higher. These temperatures present severe radiation problems to any radar antenna system mounted on the aircraft. Conventional approaches to this problem ordinarily involve the design of a radome, which protects the radar antenna system and forms part of the skin of the aircraft, that employs exotic materials and/or active cooling systems that are generally complex and expensive.
Further, to realize higher velocities and/or improved maneuvering characteristics, the aerodynamics of aircraft are being constantly improved. However, the ability to realize the improved performance is becoming increasing sensitive to anything that may affect the aerodynamics of the aircraft. Consequently, the design of a radar antenna system that is to be deployed on aircraft must consider the effects of the radar antenna system on the aerodynamics of the aircraft.
The ability of aircraft-based radar antenna systems to provide adequate information on objects in the surrounding space is also becoming increasingly difficult due to the reductions in the radar cross-sections of many of these objects. For example, as aircraft become more aerodynamic, their radar cross-section typically decreases, thereby making it more difficult for a radar antenna system to detect the aircraft. As a result, other types of sensors or detectors that operate on different principles, such as electro-optic sensors, are being used in conjunction with, or to supplement the radar. At least with respect to aircraft-based radar, the space to accommodate additional sensors, and especially those sensors that are to be used in high-speed aircraft environments, is limited. Consequently, the impact of a radar antenna system must be considered in such situations.
Another application in which the performance of present antenna systems is becoming increasingly less reliable is when there is a need for a very focused or narrow beam (transmitted or received) with low side lobes and/or high gain. A narrow beam with low side lobes is, for example, desirable in situations in which the goal is to reduce the possibility of the signals produced by either the antenna system or an object in the antenna system""s scanning area from being intercepted. High gain is also desirable in such a situation because it allows the antenna system to detect objects that produce low-power signals, or, stated another way, to operate in environments where there is a low signal-to-noise ratio (SNR). Present antenna systems that use partially overlapping beams in such situations have become increasingly less reliable as the need to operate in lower SNR environments increases.
Yet a further concern with respect to antenna systems is that as the performance requirements of the various applications has increased, the antenna systems that are designed to meet these requirements have tended to become more complex. This is of concern because increasing complexity generally results in a reduced reliability that is unacceptable in many applications.
Another concern with respect to antenna systems is that the performance may be more than is needed or acceptable for a particular application. Further, even though such an antenna system may be relatively inexpensive, the cost of the system is probably still greater than is necessary. Consequently, there is a need to assure that an antenna system is capable of achieving the necessary or acceptable performance for a particular application at a reasonable cost while also exhibiting a reduced complexity and increased reliability relative to known antenna systems.
Based on the foregoing, an object of the present invention is to provide an antenna system that addresses the high range, resolution, and field of view requirements of high-speed aircraft environments.
A further object of the present invention is to provide an antenna system that can be used in high-speed, high-temperature environments which substantially reduces the need for a radome formed of exotic material and/or the use of a complex cooling system.
Yet another object of the present invention is to provide an antenna system that is sensitive to the aerodynamics of the aircraft on which it is mounted.
Another object of the present invention is to provide an antenna system that addresses the concern for space on an aircraft and especially the space associated with the frontal surface or nose portion of an aircraft.
A further object of the present invention is to provide an antenna system that addresses applications in which the interception of an antenna system signal is undesirable.
Yet a further object of the present invention is to provide an antenna system that is relatively simple and reliable.
Another object of the present invention is to provide an antenna system that achieves the necessary or acceptable level of performance for a particular application at a reasonable cost while also striving to achieve a reduced level of the complexity and increased level of reliability relative to known antenna systems.
One embodiment of the antenna system of the present invention that is suitable for aircraft or ground-based applications includes an antenna array for receiving an electromagnetic signal and providing a plurality of signals that are representative of the electromagnetic signal. The antenna array is comprised of a plurality of antenna elements, each of which is capable of providing one signal of the plurality of signals. The antenna system further includes a beamformer for processing a first subset and a second subset of the plurality of signals provided by the antenna array to produce two xe2x80x9cbeamsxe2x80x9d. The beams are electrical signals that are representative of the electromagnetic signal received from a particular area. To be able to change the characteristics of the beams, the beamformer includes a switching network that permits selection of which antenna elements will provide the signals comprising the first and second subsets. For example, by changing the number of antenna elements whose signals comprise the beams, the beamwidths of the beams can be changed. Similarly, by changing the identity of the antenna elements whose signals make up the first and second subsets, the baseline between the two beams can be altered. This can be useful in resolving positional ambiguities. The antenna system further includes a device for cooperatively using the first and second beams produced by the beamformer to determine information on the surrounding environment, such as the location of an object. Specifically, the device can perform an amplitude comparison of the two beams to attain coarse positional information and a phase comparison of the two beams to realize an interferometer that provides fine or high resolution positional information.
In another embodiment of the antenna system that includes an antenna array, the beamformer operates such that the two beams formed by it are spatially independent or, stated another way, formed from the signals provided by two different subsets of antenna elements. Additionally, the formed beams are representative of the electromagnetic signals received by the two different subsets from overlapping areas.
Yet another embodiment of the antenna system of the present invention that is particularly adapted for applications in which the antenna system is to be mounted on a moving craft, such as an aircraft, and especially a high speed moving craft, includes an antenna array for providing a plurality of signals that has a low profile with respect to the surface of the craft to reduce any adverse impacts upon the aerodynamics of the craft. Alternatively, or in addition, the antenna array can be mounted on the side of the craft. By mounting the antenna array on the side of the craft, space for other sensors that are preferably located in the front or nose portion of the craft is increased. Furthermore, the side location also substantially reduces temperature related problems and the need for exotic materials or complex cooling systems, especially if the antenna array is also conformal with the side surface. This embodiment of the antenna system further comprises a multi-beam beamformer for producing two beams or more from the signals produced by the antenna array and a device for cooperatively using the two or more beams to provide information on the surrounding space.
The present invention also provides a method for operating an antenna system that includes the step of providing an antenna array with a plurality of elements, each element being capable of providing one of the plurality of signals. The method further includes the steps of using the signals provided by a subset of the plurality of elements to produce a first beam and using the signals provided by another subset of the plurality of elements to produce a second beam. Also included in the method is the step of cooperatively using the first and second beams to determine information on the space surrounding the antenna array. This step can involve a phase comparison of the two beams to implement an interferometer that provides fine or high resolution data on the surrounding space. This step can also include an amplitude comparison of two beams to generate coarse data on the surrounding environment. The method further includes electronically changing or switching the elements of the first and second subsets that are used to form the first and second beams. This can include changing the identity of the elements to, for example, vary the baseline between the two beams and/or changing the number of elements to alter the beamwidths of the two beams.
The present invention also provides an antenna system that is capable of using a relatively sparsely populated antenna array to achieve adequate performance while also exhibiting reduced complexity and improved reliability relative to other systems. The antenna system includes a body with a longitudinal axis and a plane that is located substantially perpendicular to the longitudinal axis. A plurality of elements that form an antenna array are attached to the body and located in a ring within the noted plane. The array is comprised of at least three elements. Three elements can enable the solution of both azimuth and elevation data unambiguously. The system further includes a beamformer for selectively using at least two of the signals provided by the antenna elements that are located in less than a 180xc2x0 range to form at least two beams. Also part of the system is a device that uses the two beams to determine the value of a parameter associated with an object located in the vicinity of the system. For instance, the two beams may be used to determine the elevational angle or azimuth angle of the object relative to the longitudinal axis of the body.
In one embodiment, the sparse population of antenna elements is achieved using broadband antenna elements.
To aid in resolving ambiguities in determining the values of one or more parameters associated with the object, the spacing between antenna elements has been carefully chosen. In one embodiment, the antenna array includes a discrete pair of elements, i.e. a pair of elements that is distinctly separated from the other elements of the array, with the spacing between the elements comprising the pair of elements chosen to aid in resolving the noted ambiguities. In another embodiment, several discrete pair of antenna elements are employed in which the distance between adjacent pairs of elements is greater than the distance between the elements comprising a pair of elements. In one case, the distance between immediately adjacent pairs of elements is substantially equal. In another case, the distance between immediately adjacent pair of elements varies. It should also be appreciated that the noted spacings between discrete pairs of antenna elements can be achieved with an array of elements that are equally spaced from one another and with certain of the elements grounded to achieve the desired spacing between discrete pairs of elements.
In one embodiment, the beamformer forms the two or more beams from the signals provided by elements that are not only within the noted 180xc2x0 range but also immediately adjacent to one another.
In a further embodiment, the beamformer forms the first beam from the signal provided by only one element and forms the second beam from the signal provided by the element immediately adjacent to the element that provides the signal to form the first beam.
In yet another embodiment, the beamformer includes multiple channels so that multiple beams can be formed simultaneously and thereby expedite the determination of unknown parameter values associated with the object relative to a beamformer with one or fewer channels.
The present invention also provides a method of operating an electrically agile, mobile antenna system to determine N unknown parameter values associated with an object located in the vicinity of the antenna system. The method includes forming a plurality of beams from the signals provided by a plurality of antenna elements that are located in a ring about the longitudinal axis of a body on which the elements are mounted. These beams are used to roughly determine the location of the object relative to body. In one embodiment, the amplitudes of the beams are compared to one another to roughly determine the location of the object. Once the location of the object is roughly known, N+1 different beams are formed, where N is the number of different parameters whose values are presently unknown or uncertain and N is at least three. For example, if the four values associated with the elevation, azimuth, and two polarization parameters for the object are unknown or uncertain, five different beams are formed from the signals provided by the antenna elements that are within a segment in space in which the object has been roughly located. This second set of beams is used to determine values for the parameters. In one embodiment, the phases of the beams are compared to one another to determine the parameter values. If high resolution is required, further beams can be generated from the signals provided by elements that are further apart, phase comparisons of the beams made, and the value of a particular parameter refined. In various embodiments, the noted beams are produced sequentially, simultaneously, or with some beams produced simultaneously and the remaining beams produced at a later time depending on the application.